In recent years, with the rapid development of the electronic and information technology industries, demands on the power sources for electronic products have increased. With the widespread use of lithium ion rechargeable batteries with their excellent properties, demands on better performance for these batteries have also increased. Lithium ion rechargeable batteries are asked to possess excellent overall properties; to simultaneously have high discharge capacity, high discharge energy, long cycle life and excellent large current discharge characteristics.
The material for the positive electrodes is one of the key elements for determining the performance of rechargeable batteries and the electrical chemical characteristics.
At present, the active materials for positive electrodes of lithium ion rechargeable batteries are mainly embedded type compounds. The more widely used ones are lithium cobalt salts such as, LiCoO2. However, LiCoO2 type materials are expensive and their sources are scarce. Therefore, compounds of lithium manganese oxides and compounds of lithium nickel oxides have been suggested as substitutes. Compounds of lithium manganese oxides have the disadvantages that batteries fabricated with these compounds as positive electrodes have lower theoretical capacities and larger ranges of decreases in their capacities during repeated charging and discharging cycles and under higher temperatures.
Although the compounds of lithium nickel oxides do not possess the weaknesses of the compounds of lithium manganese oxides, LiNiO2 possesses the same crystal structure as LiCoO2 and its properties are inferior to those of LiCoO2. Ni3+ ion, when compared with the Co3+ ion, more easily reverts back to the Ni2+ ion. Since the size of Ni2+ and the Li+ ion are similar, (rNi2+=8.7 nm, rLi+=9.0 nm), the Ni2+ ion and the Li+ ion can easily replace each other. This results in the formation of halite magnetic domains with inactive electrochemical properties that cause the lowering of the capacities of batteries when these compounds are used as active materials for positive electrodes.
In order to compensate for their weaknesses, these compounds of lithium nickel oxide are often treated by coating and/or doping with metal oxides. Among these, compounds of lithium nickel cobalt oxides doped with cobalt exhibit the best properties. Further doping of these compounds of lithium nickel cobalt oxides with metal can enhance their properties more. The specific discharge capacity of the compounds of lithium nickel cobalt oxide is generally 180 mAh/g, higher than the 140 mAh/g of the oxides of lithium cobalt. However, due to the effect of the self oxidation potential of the material, the mean voltage of the compounds of lithium nickel cobalt oxide during the discharge process is approximately 3.6V, lower than the 3.8V of the oxides of lithium cobalt. As a result, its specific energy is merely 600 mWh/g. This increase in range is limited when compared with the specific energy of the oxides of lithium cobalt (550 mWh/g). Therefore, the improvement of the overall properties of the batteries is also limited.
Due to the limitations of the prior art, it is therefore desirable to have novel fabrication methods in combination with novel materials that have good electrochemical properties such that when these novel materials are used as materials for positive electrodes of batteries, the batteries have high discharge capacities, high discharge capacity, long cycle life, and excellent large discharge current characteristics.